The following abbreviations are herewith defined, at least some of which are referred to within the following description about the prior art and/or the present invention.
3GPPThird Generation Partnership ProjectCCComponent CarrierDLDownlinkeNBEvolved Node BFDDFrequency Division DuplexingHARQHybrid Automatic Repeat RequestHSDPAHigh-Speed Downlink Packet AccessIMTInternational Mobile TelecommunicationsLTELong-Term EvolutionMIMOMultiple-Input and Multiple-OutputPDSCHPhysical Downlink Shared ChannelRRCRadio Resource ControlUEUser EquipmentULUplinkWiMAXWorldwide Interoperability for Microwave AccessLTE Rel-8 HARO Operation and Redundancy Versions
HARQ incremental redundancy is used in LTE. Thus, instead of re-transmitting the same portion of the codeword different redundancy versions are retransmitted yielding an extra gain over Chase combining. In this scheme, the receiver side (e.g., terminal side, UE side) would ideally have a full soft buffer available such that the received soft values for the entire codeword can be stored within the soft buffer. However, due to the terminal complexity and cost concerns, the soft buffer size in a terminal is limited.
Thus, for higher rate transmissions where larger codewords are sent from a transmitter (e.g., base station, eNB), the terminal may have only a limited sized soft buffer and would not able to store the complete codeword. As a result, the transmitter (e.g., eNB) and terminal (e.g., UE) must have the same understanding about the soft buffer size since otherwise the base station may transmit coded bits which the terminal cannot store or worse, it does not know these are other bits and confuses them with the bits it stores. FIG. 1 (PRIOR ART) depicts a simplified complete codeword and also how many softbits the terminal can store.
If the eNB and terminal have the same understanding about the soft buffer size then the eNB will never transmit coded bits which the terminal cannot store in the soft buffer. Instead, the eNB takes only those coded bits that are stored by terminal and uses those bits for (re-)transmissions. This situation can be depicted by the circular soft buffer 200 shown in FIG. 2 (PRIOR ART) where it should be appreciated that the complete circle corresponds to the size of the soft buffer 200 and not to the entire codeword. In the first transmission, utilizing the circular soft buffer 200 some or all systematic bits and none or some parity bits are transmitted depending on the code rate. In a retransmission the starting position is changed and bits corresponding to another part of the circumference of the circular soft buffer 200 are transmitted.
In Rel-8 LTE FDD, each terminal has up to 8 HARQ processes per component carrier and each HARQ process can contain up to two sub-processes for supporting dual-codeword MIMO transmissions. The design in Rel-8 LTE is to divide the available soft buffer equally into the configured number of HARQ processes. Each of the said divided soft buffer can be used to store soft values of the received codewords. In case of dual-codeword MIMO transmission, the divided soft buffer shall be further divided equally to store the soft values of the two received codewords. It should be noted that a codeword is an entity of a transport block that is coded and transmitted and there can be two codewords per component carrier.
More specifically, in 3GPP Technical Specification 36.212, V9.3.0 (2010-09_Section 5.1.4.1.2 “Bit collection, selection and transmission” (the contents of which are incorporated by reference herein) the soft buffer size allocation is provisioned as below:
“The circular buffer of length Kw=3KΠ for the r-th coded block is generated as follows:wk=vk(0) for k=0, . . . ,KΠ−1wKΠ+2k=vk(1) for k=0, . . . ,KΠ−1wKΠ2k+1=vk(2) for k=0, . . . ,KΠ−1
Denote the soft buffer size for the transport block by NIR bits and the soft buffer size for the r-th code block by Ncb bits. The size Ncb is obtained as follows, where C is the number of code blocks computed in section 5.1.2:
      -          N      cb        =      min    ⁡          (                        ⌊                                    N              IR                        C                    ⌋                ,                  K          w                    )      for downlink turbo coded transport channels −Ncb=Kw for uplink turbo coded transport channels
where NIR is equal to:
      N    IR    =      ⌊                  N        soft                              K          MIMO                ·                  min          ⁡                      (                                          M                DL_HARQ                            ,                              M                limit                                      )                                ⌋  
where:                Nsoft is the total number of soft channel bits . . . .        KMIMO is equal to 2 if the UE is configured to receive PDSCH transmissions based on transmission modes 3, 4 or 8 . . . .        MDL—HARQ is the maximum number of DL HARQ processes . . . .        Mlimit is a constant equal to 8.        w is the virtual circular buffer        k is an index for each bit stream        KΠ is the total number of bits for each bit stream”        
FIG. 3 (PRIOR ART) illustrates a LTE Rel-8 terminal 300 with a soft buffer 302 that has been allocated for single-codeword transmission modes or when the PDSCH transmission mode is other than mode 3, 4 or 8. In this example, the terminal 300 has a sub-buffer SB0, SB1 . . . SB7 reserved for each codeword. FIG. 4 (PRIOR ART) illustrates the LTE Rel-8 terminal 300 where the soft buffer 302 has been allocated for dual-codeword transmission modes or when the PDSCH transmission mode is mode 3, 4 or 8. In this example, the terminal 300 has a sub-buffer SB0a, SBOb, SB1a, SB1b . . . SB7a, SB7b reserved for each codeword where each sub-buffer SB0a, SBOb, SB1a, SB1b . . . SB7a, SB7b is only half the size of the previous operating case shown in FIG. 3 (PRIOR ART). Basically, each HARQ-process has a buffer and each HARQ process can contain up to two codewords, and each codeword has a sub-buffer. It is clear that the soft buffer limitation problem is particularly acute in the dual-codeword MIMO transmission operation. This soft buffer limitation reduces the effectiveness of the soft combining gains from incremental redundancy retransmissions.
A person skilled in the art will readily appreciate that a carrier has 8 HARQ process in FDD and each HARQ process can have either one or two transport blocks depending how the carrier is configured. Furthermore, each transport block is mapped to a codeword. Thus, the LTE Rel-8 FDD terminal 300 which supports only one carrier can store 8 HARQ process, if the HARQ process contains two transport blocks, i.e. also two codewords, then the soft buffer 302 for that HARQ process is divided into two sub-buffers as in FIG. 4 (PRIOR ART). In contrast, if the LTE Rel-8 terminal 300 has only one codeword per HARQ process then the soft buffer 302 is divided as shown in FIG. 3 (PRIOR ART).
Carrier Aggregation
The LTE Rel-8 standard has recently been standardized and supports bandwidths up to 20 MHz. However, in order to meet the IMT-Advanced requirements, 3GPP has initiated work on LTE Rel-10. One of the parts of LTE Rel-10 is to support bandwidths larger than 20 MHz. One important requirement for LTE Rel-10 is to assure backward compatibility with LTE Rel-8. This requirement should also include spectrum compatibility. That would imply that a LTE Rel-10 carrier, wider than 20 MHz, should appear as a number of LTE carriers to an LTE Rel-8 terminal. Each such carrier can be referred to as a component carrier (note: a component carrier can also be referred to as a cell in 3GPP). In particular, for early LTE Rel-10 deployments it can be expected that there will be a smaller number of LTE Rel-10 capable terminals when compared to many LTE Rel-8 legacy terminals. Therefore, it is necessary to assure an efficient use of a wide carrier also for legacy terminals, i.e. that it is possible to implement carriers where legacy terminals can be scheduled in all parts of the wideband LTE Rel-10 carrier. The straightforward way to obtain this would be by means of Carrier Aggregation. Carrier Aggregation implies that an LTE Rel-10 terminal can receive multiple component carriers, where the component carriers have, or at least the possibility to have, the same structure as a Rel-8 carrier. Carrier Aggregation between a base station 500 and the LTE Rel-10 terminal 502 which has a soft buffer 504 has been illustrated in FIG. 5 (PRIOR ART).
Soft buffer Operation in Carrier Aggregation
In LTE each component carrier operates with its own set of eight HARQ processes for FDD (for TDD see Table 7-1 in the 3GPP Technical Specification 36.213 V.9.3.0 dated 2010-10-03—the contents of which are incorporated by reference herein). Since the total soft buffer memory needs to be shared among component carriers, the soft buffer size per component carrier can vary depending on the number of configured component carriers and the number of configured MIMO transmission modes for each component carrier. The available soft buffer size for each codeword also depends on how the soft buffer is divided and allocated amongst all codewords. Two possible soft buffer allocation methods for the LTE Rel-10 terminal are considered and discussed next.
The first method divides the total soft buffer equally amongst the number of configured or activated component carriers. Each of the sub-buffers is then treated as a buffer in the Rel-8 operations. That is, each of the sub-buffers is then divided into the number of HARQ processes and then further divided into the number of MIMO codewords. More specifically, the soft buffer size for a codeword on the component carrier nc given by:
                                          N            IR                    ⁡                      (                          n              c                        )                          =                  ⌊                                                    N                soft                            /                              N                carrier                                                                                      K                  MIMO                                ⁡                                  (                                      n                    c                                    )                                            ·                              min                ⁡                                  (                                                            M                      DL_HARQ                                        ,                                          M                      limit                                                        )                                                              ⌋                                    (        1        )            
where Ncarrier is the number of configured/activated component carriers, nc is a index of the component carriers (i.e., nc=0, 1, . . . , Ncarrier−1), and KMIMO(j) is the number codewords configured for the component carrier j. FIG. 6 (PRIOR ART) illustrates a LTE Rel-10 terminal 502 which has a soft buffer 504 that is allocated per this method where in this example a case is considered where CC0 is configured for dual-codeword MIMO and CC1 is configured for single-codeword transmissions. While this allocation method is easy to implement, it also retains the drawback from LTE Rel-8 where the soft buffer size of the transport block on the carrier with two codewords transmission is still very limited. This limitation reduces the effectiveness of soft combining gains from incremental redundancy retransmissions.
A second method aims to provide full flexibility in soft buffer allocation. In this case, the total number of codewords from all component carriers and the associated configured MIMO modes are accounted for in the soft buffer allocation. In particular, the soft buffer is then equally divided amongst all codewords. More specifically, the soft buffer size for a codeword is given by the following:
                              N          IR                =                  ⌊                                    N              soft                                                      ∑                                  j                  =                  0                                                                      N                    carrier                                    -                  1                                            ⁢                                                          ⁢                                                                    K                    MIMO                                    ⁡                                      (                    j                    )                                                  ·                                  min                  ⁡                                      (                                                                  M                        DL_HARQ                                            ,                                              M                        limit                                                              )                                                                                ⌋                                    (        2        )            
FIG. 7 (PRIOR ART) illustrates a LTE Rel-10 terminal 502 which has a soft buffer 504 that is allocated per this method where in this example a case is considered where CC1 is configured for dual-codeword MIMO and CC2 is configured for single-codeword transmissions. It can be observed that the second allocation method improves upon the first allocation method by increasing the soft buffer size reserved for codewords in dual-codeword MIMO transmission modes. However, such an allocation freely changes the boundaries between different HARQ processes. This places a high complexity in hardware implementation and introduces many potential operation error scenarios, all of which had received extensive discussion during the design of Rel-8 LTE. In addition, in the second allocation method the soft buffer boundaries change which means operating errors in one HARQ process can spill over to another HARQ process, which magnifies the severity of any one HARQ operation error.
Reconfiguration Period
In all of the aforementioned proposals, the soft buffer sizes change if the number of component carriers or the transmission mode of one/multiple component carriers is changed altering the transport blocks per subframe. In LTE, the re-configuration of component carriers is done via RRC signaling (RRCConnectionReconfiguration message) which is not synchronized, i.e. the terminal has up to 15 ms to allocated the soft buffer and apply the new RRC configuration after reception from the eNB. However, the eNB does not know when exactly the terminal uses the updated RRC configuration. The terminal acknowledges successful application of the new configuration by transmitting RRCConnectionReconfigurationComplete to the eNB (already with the new configuration) but until the eNB has received this message it does not know for certain that the terminal is using the allocated soft buffer per the new configuration. The time before the eNB knows that the UE has applied the new configuration to the soft buffer can also be longer, if there are any of retransmission of the UL and DL RRC messages between the terminal and the eNB. In any case, if such a re-configuration changes the number of component carriers or the transport blocks per component carrier then the soft buffer size changes. As explained above, it is important that eNB and terminal have same understanding of soft buffer size since otherwise the terminal miss-interprets received bits and corrupts its soft buffer.
In view of the foregoing, it can be seen that there has been and still is a need to address the aforementioned shortcomings and other shortcomings associated with the terminal's soft buffer allocation method and the reconfiguration period where the eNB is not sure if the terminal has applied a new RRC configuration. These need and other needs are satisfied by the particular embodiments of the present invention.